Flow meters are well known devices that can be used to measure the rate of flow of a fluid such as a liquid or gas through a pipe. Known flow meters monitor various conditions such as the rate of loss of heat from a heated sensor or differential pressures to provide an output representative of flow conditions within the body of a flow meter. Each type of known flow meter has its advantages and disadvantages but with many flow meters and in particular thermal loss flow meters it is difficult to pre-calibrate a meter without a precise knowledge of the operational environment in which that meter is to be fitted. As a result, it is often the case that flow meters are delivered to the end user on the basis that after installation they will be adjusted such that the flow meter output does represent a true measure of the flow through the meter. Such an approach requires highly skilled technicians to install meters and it would clearly be highly desirable to rely upon factory-calibration to a much greater extent than is possible at present.
One problem encountered when installing flow meters is that such meters are generally inserted between flanges of a pipeline. Thus the flow meter will define an inlet of factory-determined dimensions which in use is located downstream of a passageway defined by a pipe of unknown dimensions. Even if for a particular installation the nominal internal diameter of a pipe upstream of the intended meter location is known, there is almost inevitably a discontinuity as between the meter body inlet and the pipe immediately upstream of that inlet which generates unpredictable effects within the meter body. Such unpredictable effects make it impossible reliably to calibrate a flow meter until that flow meter has been installed in its final place of use.